This invention relates generally to wireless data transmission and more particularly a system for increasing transmission capacity by using multiple subcarriers on FM broadcast radio stations.
It is well known that information can be transmitted by means of a subcarrier contained on FM broadcast radio waves. FIG. 1 shows spectral components of FM signals used in transmissions of both analog radio station broadcasts and digital paging messages. Most FM broadcast radio stations only use the baseband frequencies from 50 hertz (Hz) to 53 kilohertz (kHz) to transmit stereo program material.
In such systems, a first component 12 transmits left plus right channel audio material. A component 14 then transmits left minus right channel audio material. A stereo tone signal 18 is also transmitted at 19 kHz. A mono-channel receiver processes the analog signal from component 12. Alternatively, a dual channel stereo receiver processes both component 12 and component 14. In some geographical locations the FM baseband also includes additional subcarriers.
The remaining baseband frequencies from 53 kHz up to the legal maximum are typically available for the transmission of other data. In the United States, information can be transmitted at frequencies up to 100 kHz. In other geographical locations, such as in Europe, FM stations can only be transmitted at frequencies up to 75 kHz. Thus, different portions of the FM baseband may be available on any station depending on baseband bandwidth restrictions or the presence of other subcarrier signals.
Various issued patents and pending applications including U.S. Pat. Nos. 4,713,808 and 4,897,835 (both by Gaskill), U.S. Pat. No. 5,187,470 (King) and application Ser. No. 08/046,112, filed Apr. 9, 1993 show systems where digital data is transmitted on a FM subcarrier 16 modulated on the FM baseband shown in FIG. 1. Subcarrier 16 has a center frequency at 66.5 kHz and a bandwidth that extends from approximately 57 kHz to 75 kHz.